the National Natural Science Foundation of China(21774130);the National Natural Science Foundation of China(21574135);the Beijing Natural Science Foundation, China(2162043);the Key Research Program of the Chinese Academy of Sciences(XDPB08-2)

In recent years, non-fullerene small molecule acceptors for organic solar cells (OSCs) have attracted much research attention. Among them, perylene diimide (PDI) and its derivatives are widely investigated due to their excellent electron mobility, high electron affinity, thermal and photochemical stability, and the feasibility with which they can be chemically modified. However, the utilization of PDIs in OSCs still lags behind that of fullerenes. This is mainly because the PDI-based acceptors possess strong π–π stacking, and therefore they are inclined to form large aggregates in bulk heterojunction (BHJ) active layers. Structural modification of PDIs by disrupting their planarity plays a vital role for the application of these novel acceptors in high-performance OSCs. In this review, progresses in PDI-based small molecule acceptors for BHJ OSCs in the past three years are summarized. This work focuses on the development of molecular structures and the optimization of the power conversion efficiency (PCE) of devices. The modifications in the molecular structures are introduced according to the active PDI reaction sites, including the bay positions, ortho positions, and imide positions, to disturb the planarity and construct twisted configurations. Modifications at the bay positions are considered to be the most common and efficient; they may form PDI multimers such as dimers, trimers, and tetramers possessing quasi-3D nonplanar structures. The progress in such modifications is discussed at length. Substitutions at the imide positions are chemically simple but less effective in changing the planarity of the molecular backbone. Nevertheless, they may alter the solubility of the molecules, the film morphology, and thereby the efficiency of the devices. Functionalization of ortho positions can also effectively improve the performance of devices, but they are synthetically difficult. For conjugated PDI molecules fused at the bay positions, the properties of exciton diffusion, charge mobility and charge separation, and thereby the device performance, may be modulated by the molecular planarity, the number of the fusing unit, and the axis direction, which in turn determine the packing modes and the extent of π-extension. In summary, the photoelectric properties of PDI-based acceptors can be adjusted via various modification methods, and the relationships between the molecular structure and photovoltaic performances should be further explored.